Real talk about heartworm in Ontario.

Okay. Let’s talk about heartworm. ✋

Every single spring, the moment I bring it up in an exam room, I get the same face. You know the one. The polite half-smile, the slight head tilt, and then, “Heartworm? In Ontario? Isn’t that, like, a Florida thing?”

Honestly? I love that question. Because it tells me you’re thinking, you’re not just blindly nodding along, and you actually want to understand the “why” behind what we’re doing. So let’s get into it. No scary stats. No guilt trips. Just the real story.

First things first: what even is heartworm?

So. Heartworm is a parasite. Its fancy name is Dirofilaria immitis, but we don’t need to get cute with the Latin. The short version is that it’s a worm that, in its grown-up form, hangs out in a dog’s heart and lungs. I know. I know. Not the visual any of us wanted on a Tuesday.

Here’s how the whole thing works, in plain English:

• Step 1. A mosquito bites an infected animal. Could be another dog. Could also be a coyote or a fox, and in case you haven’t noticed, we have plenty of those wandering around Centre Wellington.
  
• Step 2. That mosquito picks up tiny baby worms (microfilariae, if you want to flex at your next dinner party) from the infected animal’s blood.
  
• Step 3. The baby worms develop inside the mosquito for about two weeks. Yes, inside the mosquito. Nature is wild.
  
• Step 4. That same mosquito bites your dog and basically injects the now-infectious larvae into your dog’s bloodstream.
  
• Step 5. Over the next six to seven months, those larvae quietly migrate up to the heart and lungs and grow into adults.

By the time a dog actually shows something is wrong (the soft cough, the getting tired faster than usual, the inexplicable weight loss), the parasite has often been chilling in there for a while. Which is exactly why we don’t sit around waiting for symptoms to play this game.

“But you literally just said it’s rare here.”

Yes. I did. And it is. I’m not going to pretend otherwise.

Ontario’s heartworm-positive rate sits at around 0.5%. In parts of the southern US, that number climbs into the 5 to 10% range, sometimes higher. So yes. Genuinely, meaningfully, lower risk here. That part is true.

But here’s the stuff that doesn’t make it into the headline statistic:

• The cases we do see in Ontario tend to cluster in the south of the province. And geographically, we’re closer to that than people think.
  
• Coyotes and foxes don’t get tested at the vet. They quietly carry the parasite and they share mosquito populations with our dogs. So the “real” number is almost certainly a smidge higher than what gets officially reported.
  
• Our summers are getting longer. Heartworm larvae need it to be at least 14°C for sustained periods to develop inside a mosquito. Our climate is shifting that window outward, year after year.
  
• Rescue and importation is a thing now. Dogs adopted from the southern US sometimes show up infected, and once they’re here, they can become a quiet source for our local mosquitoes to pick up.

So is heartworm a five-alarm emergency in Fergus? No. Absolutely not. Is it a topic worth understanding? I think so. Not because I’m trying to scare you. Just because pet parents deserve the full picture, not the headline version.

Most of the heartworm-positive dogs I’ve personally seen never left this province. It’s not common. But it’s also not theoretical.

So why do we test every single year? (Even on dogs already on prevention?)

Okay, THIS is the part that trips everybody up. Let me clear it up.

Heartworm prevention does NOT kill adult worms. 🚨 Read that again.

What prevention does is kill the immature larval stages that have been deposited by a mosquito within the past 30-ish days. So if a heartworm has already matured into an adult inside your dog’s heart, the monthly prevention isn’t going to touch it.

And here’s the part that REALLY matters. Giving heartworm prevention to a dog that already has adult heartworms can actually trigger a serious reaction. Which is the opposite of what any of us want.

So we test first. Every year. Before the prevention starts. Three reasons:

1. It confirms your dog is heartworm-negative before another season starts. No surprises.
2. It catches anything we missed early, when we have the most options.
3. It documents that the prevention is working over time. That paper trail matters more than people realize.

A quick word on the Accuplex test

The test itself is honestly nothing. A couple drops of blood. A few minutes. Done. And it’s not just heartworm. The Accuplex test we use also screens for three tick-borne diseases (Lyme, Anaplasma, and Ehrlichia) from the same sample. One blood draw, four answers. It’s the most efficient piece of testing in the building.

When exactly is heartworm season around here?

In our part of Ontario, the actual transmission window runs roughly June through November. Outside of that, it’s too cold for the parasite to develop inside a mosquito. So unlike places further south where this is a year-round conversation, our risk window is genuinely seasonal.

A couple things worth keeping in mind on the timing:

• Tick season starts WAY earlier than mosquito season. Ticks wake up any time it gets above 4°C, which around Fergus can happen as early as March. So if your dog is on a combo product, the tick coverage piece is a totally separate spring conversation.
  
• The window is creeping outward. Warmer summers, longer transmission seasons. This is something we keep an eye on every year.

What about the cats? 🐱

Real quick, because I know cat people are reading this and feeling left out.

Yes. Cats can get heartworm. It just shows up differently in them. Cats are not great hosts for this parasite (which is, weirdly, kind of good news for them), so most exposed cats actually fight the larvae off on their own. Indoor cats are at lower risk than dogs, but here’s the part nobody wants to hear: it’s not zero risk. Mosquitoes sneak inside. They always find a way.

If you have questions about your cat specifically, bring it up at your next visit. Honestly. We love when people ask. 🙌

The TL;DR

Heartworm in Ontario is uncommon. That’s the real truth, not the marketing version of the truth.

But it’s also one of those situations where the gap between “preventable” and “preventable but not prevented” is really wide. You don’t need to spiral about it. You don’t need to overhaul anything. You just need to know what’s going on, ask questions when something doesn’t make sense, and stay in a normal wellness routine with whoever your vet is.

If heartworm has been quietly nagging at you and you weren’t sure if you were doing the right thing, you probably are. And if you’re not, that’s an easy conversation we can have at your dog’s next visit. There’s no perfect protocol. The right approach depends on your specific dog, their lifestyle, where they travel, all of it.

Thanks for reading the whole way through. The fact that you wanted to actually understand this stuff is the whole reason we do what we do. 🐾